Interpretive Summary: • Problem- We know little about the factors regulating the abundance of plants in grasslands. One natural process that is likely to affect plant abundance is referred to as plant-soil feedbacks. This type of feedback is the outcome of plants culturing microbes and other things associating with their roots in the soil. This soil biology can be harmful or beneficial. If we lack an understanding of the natural processes that shape grasslands then it will be difficult to interpret the effects of many management practices intended to change the relative abundance of plants.
• Accomplishment- This study found that the dominant grass had a negative soil feedback. This means that the biology proliferating around its roots are more harmful than the soil biology associated with other plants. Other plant species were not affected by soil feedback effects. The results from this study suggest that negative soil feedbacks help to suppress the dominant grass species. This may help maintain species coexistence and promote grassland diversity and stability.
• Theoretical implications- Soil feedbacks enable plant coexistence. However, these effects manifest differently in different systems.
• Management implications- This study helps to reveal that low-input systems, like most rangelands, are likely to experience a form of “fouling of the nest”. These feedbacks help to prevent the dominant grass from excluding less competitive plants.

Technical Abstract:
Estimates of species losses and evidence of positive plant diversity-productivity relationships have spurred interest in understanding the mechanism(s) regulating species coexistence and relative abundance. Plant-soil biota feedbacks appear to affect plant diversity and community structure by either 1) suppressing dominant species, 2) causing the rarity of most plants, or 3) reducing the competitive abilities of all species. Here I use three plant-soil feedback experiments, each with 16 grassland species and representing separate sites, to differentiate the effect of soil feedbacks on prevalent mixed-grass prairie vegetation. Among all soil feedback estimates, 74% (34 of 46) were negative suggesting a general tendency for most species to have negative plant-soil feedbacks. However, only 14% (5 of 34) of these feedback point estimates were statistically significant, and two positive soil feedback estimates were detected. Two previous studies revealed plant rarity was associated with negative soil feedbacks; however, here the relative abundance of plants in the field was not positively correlated with the soil feedback of plant species for any of the three research sites. Instead, the most abundant plant species across all three sites experienced a negative soil feedback. This is compelling evidence that less abundant species are not necessarily more likely to have negative soil feedbacks than abundant species. The negative soil feedbacks reported here may still act as a fundamental driver of species coexistence, but by weakening the most dominant plant species.